In recent years, stretched films made of polyvinyl chloride resin, polystyrene resin, polyester resin or the like (the so-called thermally shrinkable film) have been widely used for the use of label packaging, cap seal, accumulated packaging, etc. having both roles for protection of glass bottles, PET bottles, etc. and for indication of products. Among the thermally shrinkable films as such, a polyvinyl chloride film has disadvantages of low heat resistance, generation of hydrogen chloride gas upon burning or cause of dioxin generation. A polystyrene film has such disadvantages that its resistance to solvents is poor, an ink having a specific composition should be used in the printing and it is to be burned at high temperature whereby a lot of black smoke with bad smell is generated upon burning. Therefore, a thermally shrinkable film of a polyester type having high heat resistance, easy burning characteristics and excellent resistance to solvents has been widely utilized as a shrinkable label and, as a result of an increase in the distribution amount of PET containers, there has been a tendency that its using amount is increasing.
Further, as to a thermally shrinkable film, that which is greatly shrunk in the widthwise direction has been commonly used in view of handling in the manufacture of labels. Therefore, the conventional thermally shrinkable polyester film has been manufactured by stretching in the widthwise direction with a high magnification for expressing a sufficient shrinking force in the widthwise direction during heating.
However, in the conventional thermally shrinkable polyester film, almost no stretching is done in the lengthwise direction intersecting at right angles to the main shrinking direction. Therefore, there is an inconvenience that its mechanical strength is low and, when it is coated on a PET bottle or the like as a label by shrinking, the label cannot be easily torn off along a perforation (or, in other words, its opening ability along the perforation is poor). Further, when the film is stretched in a lengthwise direction during the manufacture for making the opening ability along the perforation for the thermally shrinkable polyester film better, although the mechanical strength becomes high and the opening ability along the perforation is improved to some extent, the shrinking force is expressed in the lengthwise direction. Therefore, when it is coated on a PET bottle or the like as a label by shrinking, there is resulted an inconvenience that the appearance (finish upon the shrinking) becomes very bad.
In view of the above, there has been a proposal for a method of improving the opening ability along the perforation for the thermally shrinkable polyester film where an incompatible thermoplastic resin is mixed with the main materials for the thermally shrinkable polyester film (see Patent Document 1).
According to the method of the Patent Document 1, although the opening ability along the perforation for the thermally shrinkable polyester film can be improved to some extent, it is hard to say that a thermally shrinkable polyester film having a sufficient opening ability along the perforation is surely prepared thereby. Further, even when the method of the Patent Document 1 is adopted, it is only possible to stretch merely in the widthwise direction during the manufacture whereby it is impossible to manufacture the thermally shrinkable polyester film in a good yield.